dlassq()

subroutine dlassq	(	integer	N,
		double precision, dimension( * )	X,
		integer	INCX,
		double precision	SCALE,
		double precision	SUMSQ
	)

DLASSQ updates a sum of squares represented in scaled form.

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Purpose:

 DLASSQ  returns the values  scl  and  smsq  such that

    ( scl**2 )*smsq = x( 1 )**2 +...+ x( n )**2 + ( scale**2 )*sumsq,

 where  x( i ) = X( 1 + ( i - 1 )*INCX ). The value of  sumsq  is
 assumed to be non-negative and  scl  returns the value

    scl = max( scale, abs( x( i ) ) ).

 scale and sumsq must be supplied in SCALE and SUMSQ and
 scl and smsq are overwritten on SCALE and SUMSQ respectively.

 The routine makes only one pass through the vector x.

Parameters

[in]	N	N is INTEGER The number of elements to be used from the vector X.
[in]	X	X is DOUBLE PRECISION array, dimension (1+(N-1)*INCX) The vector for which a scaled sum of squares is computed. x( i ) = X( 1 + ( i - 1 )*INCX ), 1 <= i <= n.
[in]	INCX	INCX is INTEGER The increment between successive values of the vector X. INCX > 0.
[in,out]	SCALE	SCALE is DOUBLE PRECISION On entry, the value scale in the equation above. On exit, SCALE is overwritten with scl , the scaling factor for the sum of squares.
[in,out]	SUMSQ	SUMSQ is DOUBLE PRECISION On entry, the value sumsq in the equation above. On exit, SUMSQ is overwritten with smsq , the basic sum of squares from which scl has been factored out.

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Date

December 2016

Definition at line 105 of file dlassq.f.

*
*  -- LAPACK auxiliary routine (version 3.7.0) --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     December 2016
*
*     .. Scalar Arguments ..
      INTEGER            INCX, N
      DOUBLE PRECISION   SCALE, SUMSQ
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION   X( * )
*     ..
*
* =====================================================================
*
*     .. Parameters ..
      DOUBLE PRECISION   ZERO
      parameter( zero = 0.0d+0 )
*     ..
*     .. Local Scalars ..
      INTEGER            IX
      DOUBLE PRECISION   ABSXI
*     ..
*     .. External Functions ..
      LOGICAL            DISNAN
      EXTERNAL           disnan
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          abs
*     ..
*     .. Executable Statements ..
*
      IF( n.GT.0 ) THEN
         DO 10 ix = 1, 1 + ( n-1 )*incx, incx
            absxi = abs( x( ix ) )
            IF( absxi.GT.zero.OR.disnan( absxi ) ) THEN
               IF( scale.LT.absxi ) THEN
                  sumsq = 1 + sumsq*( scale / absxi )**2
                  scale = absxi
               ELSE
                  sumsq = sumsq + ( absxi / scale )**2
               END IF
            END IF
   10    CONTINUE
      END IF
      RETURN
*
*     End of DLASSQ
*

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